Optical pumping module for a laser comprising a cylindrical reflector with a polygonal base

ABSTRACT

An optical pumping module for a laser including an amplifying medium forming a bar with a circular base, at least one light source for optical pumping of the amplifying medium, and a reflector that surrounds the amplifying medium to send the light, along several passes, towards the amplifying medium and forming a cylinder with a polygonal base. The light source is placed in front of one edge of this cylinder on the opposite side of this edge from the amplifying medium. The optical pumping module may be particularly applicable to laser cutting and welding.

TECHNICAL DOMAIN

This invention relates to an optical pumping module for a lasercomprising a cylindrical reflector with a polygonal base.

A laser based on the invention can be used for industrial applications,particularly for cutting, welding, surface hardening materials and formarking of objects.

It may also be used for applications in the medical domain.

STATE OF PRIOR ART

It is known that a laser essentially comprises an amplifying medium andtwo mirrors forming a resonant cavity, the amplifying medium beingplaced between these two mirrors.

The energy necessary for operation of a laser may be suppliedelectrically, chemically or optically to the amplifying medium.

In this invention, we are interested in the third manner, in other wordswhat is called optical pumping of the amplifying medium, and moreprecisely transverse optical pumping of this amplifying medium.

In order to reduce effects that limit laser performances, it isimportant to distribute pumping light in the amplifying medium asuniformly as possible.

If nothing is done, the distribution of this pumping light is usuallynon-homogenous and often has a maximum at the source of this light.

The invention relates to a means of making this distribution homogenousand its advantages over existing techniques are the simplicity ofimplementation and construction, and therefore lower cost.

It is known that the amplifying medium of a laser absorbs all or some ofthe pumping power and that a given quantity is re-emitted in the form ofa stimulated emission, the rest of the absorbed power being transformedinto heat. This stimulated emission is called the “laser effect”.

Absorption of the pumping power follows an exponential law(Beer-Lambert's law) that is translated by a higher power being absorbedon the part(s) of the amplifying medium close to the pumping source.This creates pumping non-homogeneities; the power absorbed is not thesame at all points in the amplifying medium.

Variations in pumping at different points in the amplifying medium alsocreate local variations in the refraction index that result indeformation of the phase of the emitted laser beam.

The final consequence of this pumping non-homogeneities is a limitationto the quality of this laser beam. In particular, the deformation of thephase limits the extracted power and increases the divergence of thelaser beam.

One known method of overcoming these disadvantages is to choose anamplifying medium that is a relatively poor absorber at the wavelengthof the pumping radiation and a reflector capable of redirecting thepumping radiation that was not absorbed in the first pass, to theamplifying medium. After several passes through the amplifying medium,the pumping radiation is eventually fully absorbed.

Usually, known reflectors have a curved surface, with a lengthapproximately the same length as the amplifying medium in order toreconcentrate unabsorbed power towards the amplifying medium.

Reflectors with curved surface are described in the following documents:

T. Brand, I. Schmidt, “Design and performance of a compact 600 W cwNd:YAG rod laser system pumped by microchannel-cooled stacked diodelaser arrays”, CMA2, CLEO Europe 96

S. Fujikawa, T. Kojima and K. Yasui, “High-power high efficientdiode-side-pumped Nd:YAG laser”, published by C. R. Pollock and W.Bodsenberg, OSA TOPS vol. 10, pp. 296-299.

K. Du et al., “Neodymium:YAG 30-W cw laser side pumped by three diodelaser bars”, Appl. Opt., Vol. 37, No. 12, Apr. 20, 1998, pp. 2361-2364.

A reflector is usually machined from a metallic part that is thenpolished (to obtain an optical quality polish) and coated with areflecting layer made of gold, silver or aluminium. The quality of thereflector is better when the polishing quality is better.

Curved concave surfaces with low radii of curvature (as for the surfacesconsidered in this case) are difficult to polish correctly. Similarly,it is difficult to apply the reflecting layer uniformly.

Reflectors may also be machined from diffusing materials such as someceramics or some PTFE (Teflon [registered trademark]).

The disadvantage of these materials is their bad thermal conductivitythat makes dissipation of heat generated by residual absorption of thesematerials more difficult. Since they are also porous, their usesometimes requires an additional treatment (enamelling) when they comeinto direct contact with a cooling fluid.

Another technique for making reflectors consists of compressing adiffusing powder (for example MgO powder or BaSO₄ powder) in the spacebetween two pieces of quartz.

This type of reflector may be long, difficult and expensive to make.

The amplifying medium of a laser may usually be considered as aconvergent lens with regard to the pumping light source. This is thecase particularly for the very frequent configuration of a solidamplifying medium forming a cylindrical bar with a circular base.

This is illustrated by FIG. 1 that shows a diagrammatic cross-sectionalview of such an amplifying medium perpendicular to the X-axis of thebar.

If a reflecting plane 2 and the pumping light source 4 are placed facingeach other on opposite sides of the amplifying medium 6, the pumpingbeam 8 will be refocused on or close to this amplifying medium.

This has the disadvantage that it makes the pumping non-homogeneous inthe amplifying medium.

This type of disadvantage exists for the reflector with a polygonalcross-section described in the document by:

Y. Hirano et al., “High-average-power conductive-cooled diode-pumpedNd:YLF Laser”, Conference on Lasers and Electro-optics, vol. 6, 1998,OSA Technical Digest Series (OSA Washington D.C., 1998), pp. 103-104 inwhich the optical pumping light sources (laser diodes) are facing thereflector planes.

DESCRIPTION OF THE INVENTION

The purpose of this invention is to overcome this disadvantage ofnon-homogeneity of the optical pumping.

Its purpose is a laser optical pumping module, this module comprising anamplifying medium forming a cylindrical bar with an approximatelycircular base, at least one light source provided for transverse opticalpumping of this medium, and a reflector that surrounds this medium andthat is designed to send the light from the source along several passestowards the amplifying medium in order to homogenize pumping of themedium, this reflector forming a cylinder, the base of which is anapproximately regular polygon to create several image point-sources by akaleidoscope effect, the edges of this cylinder being parallel to theaxis of the amplifying medium, this module being characterized in thatthe light source is facing one edge of this cylinder opposite this edgewith respect to the amplifying medium, the distance between this mediumand this source being chosen to optimise the homogenisation effect ofoptical pumping.

According to a preferred embodiment of the module according to theinvention, the amplifying medium and the reflector are approximatelycoaxial. This thus facilitates manufacturing of this module.

Preferably, the length of the reflector is approximately the same as thelength of the amplifying medium. This means that the entire length ofthis amplifying medium can be used for optical pumping.

According to a first particular embodiment of the module according tothe invention, the number of faces on the reflector is odd and the lightsource is at approximately the same level as and in the middle of a faceof this reflector.

According to a second particular embodiment, the number of faces on thereflector is even and the light source is approximately on one edge ofthis reflector.

The module according to the invention may also comprise several blocks,each block comprising at least one plane face capable of reflecting thelight from the source, each face of the reflector being formed by atleast one of the plane faces of the blocks.

According to a particular embodiment of the module, the light source isplaced in an interval formed between two of the blocks such that lightemerges from the space thus formed between the plane faces of these twoblocks.

According to a first particular embodiment of the invention, the lightsource is a light emitter.

This light emitter may comprise a laser diode or a strip of laser diodesor a row of strips of laser diodes, or a stack of strips of laserdiodes, or a combination of these two geometries, this or these stripsbeing parallel to the axis of the cylinder formed by the reflector. Whenthe light emitter comprises several superposed diode strips, the spacebetween two strips preferably remains small.

In this case in which the emitter comprises a laser diode or one or morestrips of laser diodes, when the light source is placed in the intervalformed between the two blocks, the two blocks may be electricallyconducting, the laser diode or the strip(s) of laser diodes then beingelectrically powered through these two blocks.

According to a second particular embodiment of the module, each lightsource is a light propagation means, the first end of which is intendedto illuminate the amplifying medium and the second end of which isintended to receive the light from a light emitter.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be better understood after reading a description ofthe example embodiments given below, for information only and in no wayrestrictive, with reference to the attached drawings on which:

FIG. 1 diagrammatically illustrates the principle of a known opticalpumping module that has already been described,

FIG. 2 diagrammatically illustrates the principle of this invention,

FIG. 3 is a diagrammatic cross-sectional view of a particular embodimentof the optical pumping module according to the invention, using acylindrical reflector with a polygonal base with an odd number of faces,

FIG. 4 is a diagrammatic cross-sectional view of another particularembodiment of the optical pumping module according to the invention,using a cylindrical reflector with a polygonal base with an even numberof faces, and

FIG. 5 is a diagrammatic and partial perspective view of a variantembodiment of the module in FIG. 3.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

An optical pumping module according to the invention is diagrammaticallyand partially shown in FIG. 2, and comprises an amplifying medium 10forming a cylindrical bar, the base of which is circular orapproximately circular and the axis of which is denoted X.

This module also comprises a light source 12 intended for transverseoptical pumping of the medium 10.

Furthermore, this module comprises a reflector 14 forming a cylinder,the base of which is a regular or approximately regular polygon.

This cylinder is an assembly of several polished plane faces possiblycoated with a reflecting or diffusing layer. Only two adjacent faces 16and 18 are shown in FIG. 2.

The intersections of the planes of the faces of the cylinder form theedges of this cylinder and FIG. 2 shows the edge 20 corresponding tofaces 16 and 18.

The edges of the cylinder are parallel to the X-axis of the medium inthe form of a bar. Note that FIG. 2 is a cross-sectional view of themodule perpendicular to this X-axis.

According to the invention, the pumping light source 12 is placed facingthe edge 20 of the cylinder, at the opposite end of this edge withrespect to the bar shaped medium 10.

This configuration is useful for creating multiple image point-sourcesof the source 12 by kaleidoscope effect.

Reference 22 in FIG. 2 shows the light beam emitted by this source 12.

With the configuration considered, the pumping light that is notabsorbed during a first pass through the amplifying medium 10, isreflected on the corner of the reflector that faces the source 12 (inother words is reflected near the edge 20) and is therefore notrefocused towards the amplifying medium 10.

Preferably, as shown in FIG. 2, the source 12 is in the plane containingthe X-axis of this amplifying medium and the edge 20 of the reflector.This simplifies manufacturing of the module according to the invention.

Note that compared with a pumping module comprising a reflector with acurved surface, the invention has the advantage that it comprises planefaces that are easier to machine, polish satisfactorily and treat than acurved surface.

With a module according to the invention, a very uniform pumpingdistribution can be obtained taking care to place the pumping lightsource(s) at an appropriate distance from the amplifying medium.

This distance depends mainly on the divergence of the pumping beam(s)supplied by this or these sources.

The source(s) must not be too close to the amplifying medium toilluminate the medium as broadly as possible, and must not be too faraway from this amplifying medium so that it (they) does not reduce theefficiency of the laser by reducing the pumping power absorbed duringthe first pass of the pumping light.

We will now consider the examples in FIGS. 3 and 4 in which acylindrical reflector 15 with a polygonal base and an amplifying medium11 in the form of a cylindrical bar with a circular base are coaxial andare approximately the same length.

In each of these examples, the pumping module is seen in across-sectional view perpendicular to the X-axis that is common to theamplifying medium and the reflector.

In the example shown in FIG. 3, the reflector comprises an odd number offaces (more precisely 5 faces) and the pumping light source consists oflight emitters composed of strips of laser diodes 24 parallel to theX-axis.

Note that the two mirrors in the laser cavity are not shown in FIG. 3(nor on the other figures). These mirrors, that delimit this cavity, areperpendicular to the X-axis and are placed on each side of theamplifying medium.

In the example shown in FIG. 3, this amplifying medium 11 is solid. Itis located inside a tube 26 that is transparent to the pumping light.

In the interval 28 between this tube 26 and the amplifying medium 11, acooling liquid such as water is circulated by means not shown to coolthe amplifying medium.

FIG. 3 shows five groups of two metallic blocks 30 and 32 with two planefaces 34 and 36 respectively that are coplanar and the assembly of whichforms one of the faces of the reflector 15 with a pentagon-shaped baseof the module in FIG. 3, which is the reason for the five faces of thisreflector.

Each strip of laser diodes 24 is included between the two metallicblocks of the same group and is close to the faces 34 and 36 of thesetwo blocks, on the line separating these two faces.

This enables the electric power supply of the laser diodes of this strip24 by polarizing the two blocks in an appropriate manner using meansshown symbolically in FIG. 3 by the − and + signs associated with blocks30 and 32 respectively.

FIG. 3 shows the space between these two blocks 30 and 32, that ispartially occupied by the corresponding strip 24. The rest of this spaceis filled with an electrically insulating material 38.

Similarly, an element 40 made from an electrically insulating materialseparates each block 30 belonging to a given group of blocks in theadjacent group, to avoid electrical contact between these two blocks.

Furthermore, all faces 34 and 36 are polished (optical qualitypolishing) and coated with a reflecting metallic deposit (not shown)that for example may consist of a layer of gold.

The pumping light beam 42 output from each laser diode, and thatdiverges by about an angle a is equal to about 90° in the exampleconsidered, is directed towards the amplifying medium 11 and a portionof this beam penetrates into this amplifying medium 11 in which it ispartly absorbed.

Light that does not penetrate into this medium or that is not absorbedduring the first pass through it is reflected by the gold layer.

After a large number of reflections, the pumping light is completelyabsorbed either by the amplifying medium or by the gold layer.

The module in FIG. 4 is different from the module in FIG. 3 by thenumber of faces in its reflector 15; this number is even and is equal tosix in the example shown, which is why it has a cylindrical reflectorwith a hexagonal base.

In the example shown in FIG. 4, three groups of two blocks 30 and 32,and therefore three series of strips of laser diodes 24, are used, eachstrip being located on the edge common to the two plane faces of thesame group.

Each face of the reflector with a hexagonal base is composed of a planeface of one of the blocks and the angle between the plane faces of thetwo adjacent blocks is equal to 120°.

Considering the example shown in FIG. 3. again, the number of strips oflaser diodes may be less than 5 and may even be equal to 1. This numberdepends on the power required for the corresponding laser.

Thus, blocks 30 and 32 in a particular group that is not associated witha strip may all be in contact with each other (without any space betweenthem) and only each group of blocks associated with a strip must beseparated from the adjacent groups by elements made of an electricallyinsulating material 40.

Similarly, the module in FIG. 1 can only comprise one or two strips oflaser diodes.

In the case shown in FIG. 4, it will even be possible to add a strip oflaser diodes at each insulating element 40 located on the opposite sideof the X-axis from an existing strip, requiring an additional threestrips.

Furthermore, each strip of laser diodes in the examples given withreference to FIGS. 3 and 4 may be replaced by a single laser diode if ahigh power laser is not necessary, or on the other hand by severalsuperposed strips, or may be aligned along a row, or a combination ofthese two geometries may be used.

The examples in FIGS. 3 and 4 show pumping light emitters to add thislight directly into the space delimited by the reflector.

Alternatively, this light can be introduced through light propagationmeans such as the following:

dioptric systems (combinations of lenses),

catoptric systems (using mirrors) in the space delimited by thereflector to hold pumping beams parallel to the X-axis of the amplifyingmedium, these mirrors being oriented so as to direct these beamsperpendicular to this axis towards the amplifying medium,

catadioptric systems (combinations of lenses and mirrors) and

light guides.

This is illustrated in FIG. 5 in which a variant of FIG. 3 isdiagrammatically and partially shown as a perspective view.

This figure shows one of the groups of two blocks 30 and 32 betweenwhich a glass plate 44 is inserted.

The first end of this glass plate 44 is adjacent to faces 34 and 36 ofthese blocks 30 and 32.

The second end of this plate 44 is optically coupled to a strip of laserdiodes 46 that is controlled by means not shown.

This strip emits pumping light 42 that is then transported by the plate44 and exits through the first end of this plate to illuminate theamplifying medium (not shown).

What is claimed is:
 1. Laser optical pumping module, comprising: anamplifying medium forming a cylindrical bar with an approximatelycircular base; at least one light source provided for transverse opticalpumping of the medium; and a reflector surrounding the medium andconfigured to reflect light from the at least one light source towardthe medium along several passes to homogenize pumping of the medium, thereflector forming a cylinder, the base of which is a substantiallyregular polygon to create several image point-sources by a kaleidoscopeeffect, edges of the cylinder being parallel to a centerline axis of themedium, wherein the at least one light source is placed in front of oneedge of the cylinder on an opposite side of the one edge from themedium, a distance between the medium and the at least one light sourcebeing configured to optimize the homogenization effect of opticalpumping.
 2. Module according to claim 1, in which the amplifying mediumand the reflector are substantially coaxial.
 3. Module according toclaim 1, in which a length of the reflector is approximately a samelength as a length of the amplifying medium.
 4. Module according toclaim 1, in which a number of faces on the reflector is odd and the atleast one light source is at approximately a middle of a face of thereflector.
 5. Module according to claim 1, in which a number of faces onthe reflector is even and the at least one light source is approximatelyon one edge of the reflector.
 6. Module according to claim 1, furthercomprising plural blocks, each block comprising at least one plane faceconfigured to reflect the light from the light source, each face of thereflector being formed by at least one of the plane faces of the blocks.7. Module according to claim 6, in which the at least one light sourceis placed in an interval formed between two of the blocks such that alight emerges from the space thus formed between the plane faces of thetwo blocks.
 8. Module according to claim 1, in which the light sourceincludes a light emitter.
 9. Module according to claim 8, in which thislight emitter comprises at least one of a laser diode or a strip oflaser diodes, a row of strips of laser diodes, and a stack of strips oflaser diodes, parallel to an axis of the cylinder formed by thereflector.
 10. Module according to claim 7, in which the light sourceincludes a light emitter and the light emitter comprises at least one ofa laser diode, a strip of laser diodes, a row of strips of laser diodes,and a stack of strips of laser diodes, parallel to an axis of thecylinder formed by the reflector, and in which the two blocks areelectrically conducting and the laser diode, the strip, the row, or thestack is electrically powered through the two blocks.
 11. Moduleaccording to claim 1, in which the light source is a light propagationmeans, a first end of which is configured to illuminate the amplifyingmedium and a second end of which is configured to receive light from alight emitter.
 12. A laser optical pumping module, comprising: anamplifying medium; a light source; a plurality of reflectors; and aplurality of insulators configured to be positioned between each of theplurality of reflectors, wherein the amplifying medium is shaped as acylindrical bar having a substantially circular base, wherein the lightsource is configured to provide light for optically pumping theamplifying medium, wherein the reflector is configured to reflect lightfrom the light source toward the amplifying medium, thereby homogenizingpumping of the amplifying medium, and the reflector is shaped as acylinder having a base shaped as an approximately regular polygon, edgesof the cylinder being parallel to a centerline axis of the amplifyingmedium, wherein the light source is placed in front of one edge of thecylinder on an opposite side of the one edge from the amplifying medium,a distance between the amplifying medium and the light source beingchosen to optimize homogenized pumping of the amplifying medium. 13.Module according to claim 12, in which the amplifying medium and thereflector are substantially coaxial.
 14. Module according to claim 12,in which a length of at least one of the plurality of reflectors isapproximately a same length as a length of the amplifying medium. 15.Module according to claim 12, in which a number of faces on theplurality of reflectors is odd and the light source is at approximatelya middle of a face of each of the plurality of reflectors.
 16. Moduleaccording to claim 12, in which a number of faces on the plurality ofreflectors is even and the light source is approximately on one edge ofeach of the plurality of reflectors.
 17. Module according to claim 12,further comprising plural blocks, each block comprising at least oneplane face configured to reflect the light from the light source, eachface of the reflector being formed by at least one of the plane faces ofthe blocks.
 18. Module according to claim 12, in which the light sourceis placed in an interval formed between two of the blocks such that alight emerges from the space thus formed between the plane faces of thetwo blocks.
 19. Module according to claim 12, in which the light sourceincludes a light emitter.
 20. Module according to claim 19, in whichthis light emitter comprises at least one of a laser diode or a strip oflaser diodes, a row of strips of laser diodes, and a stack of strips oflaser diodes, parallel to an axis of the cylinder formed by thereflector.
 21. Module according to claim 18, in which the light sourceincludes a light emitter and the light emitter comprises at least one ofa laser diode, a strip of laser diodes, a row of strips of laser diodes,and a stack of strips of laser diodes, parallel to an axis of thecylinder formed by the reflector, and in which the two blocks areelectrically conducting and the laser diode, the strip, the row, or thestack is electrically powered through these two blocks.
 22. Moduleaccording to claim 12, in which the light source includes a lightpropagation means having a first end configured to illuminate theamplifying medium and a second end configured to receive light from alight emitter.